Cathepsin S is a lysosomal cysteine protease expressed mainly in antigen-presenting cells such as dendritic cells, macrophages, and B cells, and is responsible for the degradation of the invariant chain bound to major histocompatibility complex class II (MHC class II) molecules at the time of generation. The MHC class II molecules bind to a self or non-self peptide incorporated extracellularly, and induces secretion of various cytokines by presenting the self peptide or the non-self peptide to CD4-positive T cells. It was confirmed that inhibition or deletion of the cathepsin S inhibits loading of an antigenic peptide to the MHC class II molecules, and furthermore, suppression of antigen presentation to the CD4-positive T cells lowers immune response against foreign antigens (“Immunity”, 1999, vol. 10, No. 2, p. 207-217). It is considered that in a case of autoimmune disease such as SLE, the above-described antigen presentation occurs with respect to a pathogenic self peptide, and therefore it is considered that there is a high possibility of a cathepsin S inhibitor being useful for treating autoimmune disease (“Journal of Clinical Investigation”, 1998, Vol. 101, No. 11, p. 2351-2363).
Accordingly, it is expected that the cathepsin S inhibitor is promising as an agent for preventing and/or treating autoimmune disease including SLE and lupus nephritis, or an agent for preventing and/or treating allergies, or graft rejection of an organ, bone marrow or tissue.
Patent Document 1 discloses that a compound of the formula (A) exhibits the cathepsin S inhibitory effect and is useful for treating various metabolic disease or immune disease such as SLE.

(Refer to this publication for the symbols in the formula.)
Patent Document 2 discloses that a compound of the formula (B) exhibits the cathepsin S inhibitory effect and is useful for treating various metabolic disease or immune disease such as SLE.

(Refer to this publication for the symbols in the formula.)
Non-Patent Document 1 discloses that a compound of the formula (C) interferes with the progression of SLE and lupus nephritis.

Patent Document 3 discloses that a compound of the formula (D) exhibits the cathepsin S inhibitory effect and is useful for treating diabetes and the like.

(Refer to this publication for the symbols in the formula.)
Patent Document 4 discloses that a compound of the formula (E) exhibits the cathepsin S inhibitory effect and is useful for treating diabetes and the like.

(Refer to this publication for the symbols in the formula.)